A widely carried out therapy for stenosis of blood vessels, in particular stenosis of the coronary artery, which is a cause of myocardial infarction, angina pectoris and so on, is to expand the stenosed part using a catheter having a balloon disposed on the tip thereof, which is known as a PTCA (percutaneous transluminal coronary angioplasty) balloon catheter. Describing this technique in more detail, first a hollow φ2 mm to φ3 mm catheter called a guiding catheter for leading in the PTCA balloon catheter is led into the aorta, and the tip thereof is disposed at the entrance of the coronary artery. Next, a wire of outside diameter φ0.010″ (0.254 mm) to 0.018″ (0.457 mm) which is called a guide wire and fulfills a role of guiding the PTCA balloon catheter is led into the guiding catheter, and is passed through the stenosed part of the coronary artery. Then, the PTCA balloon catheter having the balloon disposed on the tip thereof is led in along the guide wire as far as the coronary artery, is similarly passed through the stenosed part, and the balloon part of the PTCA balloon catheter is disposed in the stenosed part. The balloon is then expanded using high-pressure physiological saline, contrast medium or the like, thus forcibly opening up the stenosed part.
However, there is a large problem with this PTCA therapy in that after the therapy, restenosis, i.e. repeated stenosis, occurs with a probability of approximately 40% within a short time period of 3 to 6 months. It has been shown that restenosis is caused by the blood vessel walls being damaged through the forcible expansion of the blood vessel by the balloon, and then smooth muscle cells proliferating excessively during the subsequent healing process.
As a countermeasure for this problem, it has been found that the incidence rate of restenosis can be reduced to 20% or less by leaving behind a metal tube called a stent after the balloon expansion, but from a clinical perspective there is an urgent need to further reduce this figure.
Recently, in Europe and America there has thus been progress in the clinical application of intravascular radiation therapy as a restenosis prevention method, and the results thereof have attracted attention. In some clinical trials, the results have been that the probability of restenosis occurring could be reduced down to approximately 7%. It is said that the reason for this is that if a suitable dose of radiation is irradiated onto the lesioned part after the balloon expansion, then cell proliferation during the healing process can be inhibited.
Currently, as a therapy system used in this application, there is a catheter system for intravascular radiation therapy. This is used after the expansion of the lesioned part using a PTCA balloon catheter, or after the placement of a stent. More specifically, after the PTCA treatment, the PTCA balloon catheter is pulled out of the body, and then a catheter for intravascular radiation therapy having a hollow tubular shaft sealed at the tip is led in as far as the lesioned part. As disclosed in U.S. Pat. No. 5,199,939, a wire having a radiation source at the tip thereof (a radioactive wire) is then passed through the lumen of the tubular shaft having the sealed tip, and is led as far as the lesioned part, and then radiation is irradiated from the radiation source for the required time. In general, a dose of approximately 20 to 30 Gy is irradiated. After the irradiation has been completed, the radioactive wire is pulled out of the body (withdrawn), and then the catheter for intravascular radiation therapy is also pulled out of the body, thus completing the therapy. In general, the leading in and withdrawal of a radioactive wire are carried out through remote automatic operation of a remote loader/unloader to prevent the surgeon from being exposed to radiation, with this often being done in the field of cancer therapy in particular. There are disclosures regarding this in U.S. Pat. No. 5,199,939, U.S. Pat. No. 5,302,168, U.S. Pat. No. 5,213,561, Published Japanese Translation of PCT Application No. 10-507951, and so on.
In recent clinical application, awareness has become great of the necessity of irradiating the blood vessel walls uniformly, and the necessity of securing blood flow from the proximal side to the distal side (peripheral side) during the therapy or during the irradiation, i.e. the necessity of a perfusion mechanism. Regarding irradiating the blood vessel walls uniformly, if the radiation source shifts away from the center of the blood vessel cross section when the radiation source is positioned at the lesioned part in the blood vessel, then the blood vessel wall that is too close to the radiation source will be irradiated excessively, resulting in necrosis of the blood vessel, an aneurysm or the like. Conversely, a dose of radiation sufficient for inhibiting smooth muscle proliferation will not reach the blood vessel wall far from the radiation source. The reason for this is that the energy of the radiation irradiated from the radiation source drops with distance from the radiation source. In the catheter system for intravascular radiation therapy, a mechanism is thus required that enables the blood vessel walls to be irradiated with a uniform dose by having a so-called centering function of positioning the radiation source in the center of the blood vessel cross section or the center of the cross section at the stenosed part.
Regarding the other requirement of securing blood flow to the distal side (peripheral side) blood vessels, i.e. the necessity of a perfusion mechanism, the required irradiation time is long, being approximately 5 to 10 minutes in the case that the radiation used is β-rays, and approximately 10 to 30 minutes in the case that the radiation used is γ-rays. In the case that such a long time is required as the irradiation time, if the coronary artery blood were not to flow to the peripheral coronary artery blood vessels during the irradiation, then the myocardial cells in peripheral parts would become ischemic, causing serious symptoms such as angina. A mechanism that allows blood to flow to the peripheral blood vessels at all times while irradiating the radiation and while carrying out centering during the irradiation, i.e. a perfusion mechanism, is thus required in the catheter system for intravascular radiation therapy.
Regarding the above, Published Japanese Translation of PCT Application No. 9-507783 discloses a number of mechanisms that simultaneously realize a centering function and a perfusion function. One such centering mechanism consists of spiral lobes, in which a balloon is disposed wound around a catheter tube in a spiral fashion. The radiation source is led in as far as the tip of the catheter shaft, and then the spiral balloon is expanded, whereby the radiation source can be positioned approximately in the center of the blood vessel cross section. Moreover, due to the spiral shape of the spiral balloon, during expansion blood is fed through the grooves from the proximal side to the distal side of the balloon.
However, in the case of these spiral lobes (the spiral balloon), unless special measures are adopted, problems and inconveniences such as the following arise.
A first problem is that if the thickness of the balloon is constant in the circumferential direction after molding, then in the balloon's natural expanded state, the balloon will not be a spiral shape, but rather will be a straight shape. Consequently, when one attempts to wind the balloon in a spiral fashion around the catheter shaft and fix the balloon using an adhesive or the like, the balloon tries to return to its natural straight shape, and hence the fixing of the balloon in a certain position on the surface of the catheter shaft is difficult, i.e. there is a difficulty in terms of fixing the balloon precisely in position, and hence reproducibility is poor. This is a big disadvantage from a manufacturing perspective in particular in the case of using an adhesive having a long hardening time. Moreover, if the spiral balloon is not fixed precisely in position, i.e. if places exist where the groove width is too large, then the precision of the centering will tend to become poor, which is a serious problem from a clinical perspective.
A second problem is that when fixing the straight balloon onto the catheter shaft such that the balloon goes into a spiral state, it is necessary to twist the balloon slightly forcibly. Due to reaction, stress thus arises in the balloon such as to twist the shaft back. With a catheter for intravascular radiation therapy, a radiation source wire passes through (is delivered through) the lumen of the catheter; if the shaft is twisted, then the shaft lumen will deform, and hence there will be resistance when the radiation source passes through the catheter lumen, or in the worst cases the radiation source will not pass through the catheter lumen. In such a case, the time for which the radiation source is in the twisted part of the shaft will become long, and hence the patient's exposure to radiation at this part will increase, which is not only a large problem in terms of safety, but moreover it will no longer be possible to irradiate the region to be treated with radiation effectively.
A third problem is that when the balloon has been expanded, there will be a large stress in the balloon due to trying to return to the natural straight state, and in the case that the expansion pressure is high, there will be a risk of the balloon dropping off the shaft due to this stress. These may be serious problems from a clinical perspective.
A fourth problem is that before the balloon is expanded, and when the balloon has been expanded and then contracted, there are large level differences on the outer surface. In particular, the level differences on the balloon part after the balloon has been expanded and then contracted are marked, resulting in a large resistance when moving through a blood vessel. Moreover, in the worst cases, the inside of the blood vessel may be damaged.
Moreover, Japanese Patent Application Laid-open No. 10-179751 also discloses a number of mechanisms that simultaneously realize a centering function and a perfusion function. As the centering mechanism, there is a centering balloon that has at least two expandable spokes, and this is installed on the outer surface near to the tip of the catheter. By expanding the centering balloon symmetrically, a radiation source disposed in the catheter is centered in a blood vessel. However, problems and inconveniences such as the following arise with the structure disclosed in Japanese Patent Application Laid-open No. 10-179751.
A first problem is that the shape of the centering balloon consists of spokes that are long in the axial direction; when the centering balloon is expanded, the balloon tries to return to its natural straight shape, and hence this part becomes rod-like and hard. In the case for example that the lesioned part to be treated is an extremely curved blood vessel, the balloon may lose out to the curvature of the blood vessel and may thus not expand sufficiently to realize perfusion, and moreover the blood vessel may be irradiated excessively with radiation at the part where the balloon has not expanded sufficiently, which may cause necrosis of the blood vessel, an aneurysm or the like. Moreover, the centering balloons, two or more of which are provided as described above, will not expand uniformly, and hence it will not be possible to realize the centering of the radiation source, and as a result part of the lesioned region to be treated may be irradiated excessively with radiation, which may cause necrosis of the blood vessel, an aneurysm or the like. Furthermore, upon expanding the centering balloon, a force acts to straighten out the curved blood vessel, and with a peripheral blood vessel in particular the blood vessel may be damaged.
A second problem is that because a centering balloon having at least two expandable spokes is installed on the outer surface close to the tip of the catheter, even before expansion, the structure is such that there are large level differences on the outer surface of the balloon. The catheter must proceed along a narrow, curved blood vessel as far as the region to be treated, but if there are large level differences around the catheter, then the ability of the surgeon to maneuver the catheter will be impaired, and it may not be possible to dispose the catheter in the region to be treated, and moreover the inside of the blood vessel may be damaged by the level differences. Furthermore, when the pressure is reduced and the centering balloon is contracted after having been expanded, then the state becomes such that the balloon sticks outwards in wing shapes, i.e. such that there are yet bigger level differences on the outer surface of the balloon, and hence there will be a large resistance when moving through the blood vessel. Moreover, in the worst cases, the inside of the blood vessel may be damaged.